Method and apparatus for solar-greenhouse production and harvesting of micro-algae

ABSTRACT

A micro-algae growing method employs a greenhouse having a transparent, double-pane roof structure and containing an open-top receptacle for a bed of aqueous micro-algae medium, which roof structure and receptacle are substantially coextensive and rectangular. A series of remotely controllable nozzles, capable of producing thin, sheet-like discharges, withdraw the aqueous liquid medium from subsurface regions along the length of the bed and discharge it into the overlying space, thus optimally exposing the medium to solar radiation passing through the roof structure and thereby promoting micro-algae growth. Ambient air, heated during passage through channels in the transparent roof structure, is used in a second greenhouse for lofting small droplets that comprise sprays of the concentrated micro-algae medium received from the first greenhouse, thus promoting evaporation of free water and cooperating in harvesting of micro-algae product.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 12/650,618, filed Dec. 31, 2009, now abandoned which in turnclaims the benefit of U.S. Provisional Patent Application No.61/204,172, filed Jan. 2, 2009, the contents of which applications areincorporated hereinto, by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and means of controlling theabsorption of solar energy by a liquid contained in a greenhouse bymeans of varying the breakup and solar exposure of the liquid bylinearly deforming, spraying or atomizing it in application to massproduction and harvesting algae, desalination of water and extraction ofcarbon dioxide from flue gas.

2. The Current Needs

The worldwide discussion of the need for a practicable means ofoffsetting global warming by reducing emission of carbon dioxide hasfocused attention on sequestering the significant quantities of carbondioxide released from coal fired power plants as the primary means ofoffsetting global warming. Considerable effort is currently underway, orunder consideration, to develop methods of separating the carbon dioxidefrom the other constituents of the combustion flue gas. Its separationand collection requires its liquefaction for transportation or storage.One of the methods being studied, for sequestering the large quantitiesof CO₂ that would be collected, is to transport it to sites suitable fordeep-earth drilling and long-term storage in known underground cavitiesusing deep earth drilling. It is recognized to be a costly solution,however.

An alternative solution is to utilize the CO₂ by its absorption in thenatural process of growing algae with sunlight. This method is currentlyunder development in various stages ranging from laboratory studies andpilot scale tests to algae growing farms. The latter stage involves theuse of large capacity growth beds, covering many acres, fed by sourcesof naturally growing algae culture plus nutrient-enriched solutions.These are blanketed with carbon dioxide enriched air under transparentcanopies exposed to sun light. The growth rate of the algae is subjectto the naturally varying conditions of sunlight and heat, as well as thevarying and limited depth-penetration, into the nutrient solution, ofthe solar rays and carbon dioxide.

Methods currently used to offset the growth limiting factors involvesolution stirring, including paddlewheel mixing, and bubbling of theair-CO₂ mixture up through transparent (glass) columns of algaesolution. The growth also requires alternating periods of darkness andlight exposure. Improved means of controlling the several variables thateffect growth can serve to increase process efficiency andcost-effectiveness.

The prevalence of micro-algae growth in coastal sea waters has adverselyaffected the economies of marine industries, e.g., the destruction ofclam beds by “brown tides.” A low cost method of collecting,concentrating and harvesting the algae can overcome the problem.

The increasing shortages of water in developing countries point to theneed of sources of desalinated sea water. Current methods of producingpotable water by distillation or osmosis are costly in terms of bothcapital and operating expense. A low cost method that includes solarenergy evaporation and condensate collection can provide a world-widebenefit. Investigations have been undertaken of the feasibility ofabsorbing carbon dioxide from flue gas into aqueous mixtures of reactivechemicals. Considerable interest has been shown in its well knownreaction with magnesium hydroxide slurry to form the carbonates. Bysubsequently heating the reaction-product mixture, concentrated carbondioxide is evolved and collected.

The magnesium hydroxide slurry is then recycled for reuse. A proposedmeans of employing this reaction in flue gas cleaning has involved theuse of a conventional wet scrubber for the absorption, followed bycirculating the slurry to a steam heated reaction vessel to drive offthe CO₂, Major questions pursuant to its industry adoption include thereaction time required for absorption and the energy required to extractthe CO₂.

BACKGROUND TECHNICAL SUPPORT

An element of the apparatus utilized in the current invention employsthe method and teachings of expired patent, “Variable Gas Atomization,”which was issued to this inventor on Feb. 9, 1982, (Reference 1). Asutilized herein, variable gas atomization (VGA) refers to the method anddesigns of compressed air atomizing nozzles as described in Reference 1and as described in modified form in Reference 2. Specifically, itrefers to the use of nozzles that linearly deform the internally flowingliquid into a thin, flat sheet. This is done by employing cantilevereddividing walls that are deflected by the pressure difference between theliquid and compressed air to form thin liquid sheets of variablethickness, and typically ranging from somewhat less than 0.001″ to0.010″ (25 to 250 microns). By varying the pressures and quantities ofeither the liquid of the compressed air flowing on both sides of theliquid sheets as the air and water pass through a converging, linearnozzle exit, the exiting sprays may be varied in form from that of flatsheets that break up into coarse droplets as they settle to that of morefinely atomized droplets. The range of variation of sheet thickness andultimate droplet size depends upon the thickness and cantilevered lengthof the walls dividing the liquid and air feed channels, and the range ofpressure difference variation.

REFERENCES

-   1. Walsh, Jr., William A., “Variable Gas Atomization,” U.S. Pat. No.    4,314,670, Feb. 9, 1982.-   2. Ellison, William, Ellison Consultants, Monrovia, Md., William A.    Walsh, Jr., VGA Nozzle Company, Manchester, N.H., Prof, Dr. Adnan    Akyarli, Managing Director AKOKS, Izmir, Turkey and Prof. Dr. Aysen    Muezzinoglu, Pres. TUNCAP, Izmir, Turkey, “Commercial Application in    High Efficiency FGD of Sorbent Injection with Flue Gas    Humidification,” Sixteenth Annual International Pittsburgh Coal    Conference, Oct. 11-15, 1999, Pittsburgh, Pa.

SUMMARY OF THE INVENTION

It is the broad object of the present invention to provide a method forgrowing micro-algae under natural conditions, which method is ofsubstantial benefit from both ecological and also energy-utilizationstandpoints.

It is a further object of the invention to provide a method having theforegoing features and advantages, which is augmented so as tofacilitate the harvesting of micro-algae product in a highly desirablemanner.

It has now been found that the foregoing and related objects of theinvention are attained by the method for growing micro-algae, comprisingthe steps:

providing a greenhouse that is generally rectangular, viewed in plan,and having roof structure that is also generally rectangular, that istransparent to solar radiation, and that is of double-pane constructionto define at least one channel through which ambient air can pass to beheated by absorption of solar energy, the greenhouse containing open-topcontainment means, comprised of at least one receptacle, for thecontainment of a substantially continuous liquid bed and having an inletadjacent one end and an outlet adjacent an opposite end, the containmentmeans being spaced a substantial distance beneath the transparent roofstructure and extending along substantially the full length and widththereof, with the greenhouse defining an enclosed space thereabove;

introducing into the containment means a quantity of an aqueous liquidmedium that contains micro-algae organisms and is suitable for growingmicro-algae therein, the quantity of the aqueous liquid mediumintroduced providing a bed that fills the containment means to a depthsufficient to provide a subsurface bed region that is dark, relative tothe surface of the bed, and that extends at least along substantiallythe full length of the containment means;

at least periodically adding to the containment means, at the inlet, afresh supply of the aqueous liquid medium and withdrawing, at theoutlet, a volume of the aqueous liquid medium in which the concentrationof micro-algae has been increased substantially from the concentrationof micro-algae in the fresh supply of the aqueous liquid medium;

repeatedly or continuously drawing quantities of the aqueous liquidmedium from the subsurface region of the bed and spraying suchquantities of aqueous liquid medium into the enclosed space, at numerouslocations spaced longitudinally from one another, using a multiplicityof nozzles that are constructed to enable characteristics of the spraydischarge to be varied by means controlled remotely from the nozzles,the nozzles effecting discharge of the aqueous liquid medium frompositions above the bed surface and in the form of thin, substantiallyflat sheets that are oriented substantially horizontally, or at a smallangle of inclination, relative to horizontal;

causing ambient air to flow through the at least one channel of the roofstructure so as to permit the air to absorb a substantial portion of thesolar radiation impinging on the roof structure and thereby to produce asupply of heated air exiting therefrom; and

controlling the spray discharge characteristics, the quantity of theaqueous liquid medium sprayed, and the duration and frequency of thecontinuous or repeated spraying, for optimization of the exposure ofmicro-algae in, and drawn from, the bed to solar energy passing throughthe roof structure and to induce mixing of the aqueous liquid medium inthe bed, thereby promoting micro-algae growth and, in turn, increasingthe concentration of micro-algae in the bed, control of the spraydischarge characteristics being such as to cause at least about 90weight percent of the aqueous liquid medium issuing from the nozzles toreturn to the bed, which result promotes minimization of contact of thespray discharge with greenhouse roof and wall structures.

In preferred embodiments of the method, the angle of inclination atwhich the substantially flat sheets of spray discharge from the nozzlesis about zero to 20 degrees (relative to horizontal), and the sheets areabout 0.01 to 0.1 inch thick; liquid medium returning to the bed willusually consist essentially of streams, or droplets having diameters ofat least about 20 microns. The bed of aqueous liquid medium willnormally be about one to four feet deep, and the subsurface bed regiondefined will lie at least about 0.25 inch below the bed surface. In atypical situation, the concentration of micro-algae organisms in theaqueous liquid medium introduced into the containment means, and in thefresh supply of the aqueous liquid medium added thereto, will be about0.01 weight percent, and the concentration of micro-algae organisms inthe aqueous liquid medium withdrawn from the containment means will beabout two percent by weight.

In especially preferred embodiments of the method, the flow of ambientair through the channel or channels of the roof structure is controlledso as to control the amount of solar energy that pass therethrough, foroptimization of micro-algae growth in the greenhouse. Air and carbondioxide will normally be introduced into the enclosed space above thecontainment means, directly and/or through the multiplicity of nozzlescontained in the greenhouse, and at least one nutrient, effective forpromotion of micro-algae growth, will be introduced into the bed ofaqueous liquid medium. The temperature of the aqueous liquid medium inthe bed, and of the environment within the enclosed space, will becontrolled to a value of about 60° to 120° Fahrenheit, although for somespecies of micro-algae a temperature of about 68° to 72° Fahrenheit willdesirably be maintained. Relative humidity, in the environment withinthe enclosed space, will desirably be maintained at a value of at leastabout 80 percent.

Additional objects of the invention are attained by the provision of amethod that additionally enables harvesting of a micro-algae product. Todo so, the method will include a further step of evaporating free waterfrom micro-algae contained in the volume of the aqueous liquid mediumwithdrawn from the containment means, with the supply of heated airexiting from the greenhouse roof channel being employed in effectingthat step. To facilitate harvesting, a second greenhouse will preferablybe provided in which the evaporating step is effected, with the onechannel of the roof structure of the first-mentioned greenhouse beingoperatively connected to the second greenhouse for the delivery of thesupply of heated air thereto. The second greenhouse will most desirablycontain a second multiplicity of spray nozzles, which are supplied bythe aqueous liquid medium withdrawn from the outlet of the containmentmeans. Heated air, obtained from the first greenhouse, is being employedfor lofting of droplets discharged from the second multiplicity of spraynozzles, being facilitated by ensuring that no more than about 50 weightpercent of the so-discharged droplets have diameters larger than about40 microns.

Taking a more comprehensive view, the concepts disclosed herein serve tooptimize the utilization of solar energy by means of a variable form,and controllable degree, of atomization. The concepts are utilized topromote and optimize the mass production of micro-algae together withits collection as an industrially applicable dewatered product; they mayalso be used in the production of desalinated water, such as forindustrial applications, and for the extraction of CO₂ from flue gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a system including two adjoinedgreenhouses comprised of beds containing liquids with transparent panelcovers set at angles relative to the solar latitude and seasonal anglesuited to the particular operations described herein.

FIG. 2 shows plan and elevation views of a system including a modifiedflue gas duct comprised of a bed containing a re-circulated liquid forabsorbing CO₂ and an associated greenhouse for solar extraction of theabsorbed CO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENTS PERTAINING TO THIS INVENTION

Algae Production

FIG. 1 shows an assembly of two adjoined greenhouses, generallydesignated as items 100 and 200, as typically employed herein for thesolar production and solar harvesting of micro-algae and/ordesalination. Greenhouse 100 is used for growing and concentratingmicro-algae.

Greenhouse 200 is used for harvesting the algae by atomizing itsconcentrated dispersion and evaporating the fine droplets to drynessplus collection of algae, together with dried nutrient and salts, byfiltration. Pertinent features of greenhouse 100 include algae-suspendednutrient solution mixture M, algae-containing solution bed 101, of widthW, depth D and length L, outer roof coverings 102 and 103, and innerroof coverings 104 and 105. Algae bed depth D is generally shallow andof the order of 1 to 4 feet so as to not produce the extended period oflight exclusion that results with increasing depths. Width W is selectedto suit construction costs and, as illustrated, would generally be ofthe order of 30 to 70 ft. Length L is proportional to the scale of algaeproduction. It could be comprised of individual section lengths of theorder of 100 feet, more or less, and could extend to cover many acres.Other ratios of length to width may be chosen to suit the availableterrain. Outer roof coverings 102 and 103 and inner roof coverings 104and 105 consist of two layers of transparent panels (such as glass orplastic) separated by spaces 106 and 107 to allow passage of air. Roofcoverings 102 and 104 are oriented in a southerly direction (in northernlatitudes) and tilted at a suitable angle in order to generally maximizethe transmission of solar energy.

Dilute algae-water suspension feed F is drawn from a naturally growingsource (pond, stream or sea bed), screened of foreign matter anddelivered into one side of the growing bed (or bed section) at intervalsalong its extended length. Production may also be initiated by feedingfrom specific laboratory grown strains of algae. Growth promotingnutrients N are added to feed F as needed. Algae-nutrient mixture M isdrawn continuously from bed 101 by metering pumps 108 and delivered tolinear VGA nozzles 109 where it is atomized for exposure to solar energyand carbon dioxide enriched air. Mixture M issues from linear VGAnozzles 109 in the form of thin, extended plume P issuing mostly in theform of thin sheets that break up into coarse spray droplets thatquickly settle into bed 101 after a brief exposure to solar energy. Thenozzles are operated in a mode to specifically produce coarseatomization, and are designed with features that enable considerablevariation in sheet thickness and droplet size. By varying the degree ofliquid break-up, the exposure to solar flux is controlled and varied soas to maximize the growth rate as the solar energy varies. Moderatelycompressed (generally in the range of 5-30 psig.) atomizing air C andsecondary, blower air 5 are delivered to nozzles 109 to assist in theformation and control of the degree of atomization of liquid into sprayplume P issuing from the nozzles. Additional, tertiary gas mixture G,consisting of air and CO₂, (such as flue gas) at approximately ambientpressure, may be delivered separately through nozzles 109 to mix withplume P. CO₂ may be added to air flows C and 5 to provide intimatecontact with spray droplets. Although only two are shown in FIG. 1, itwill be understood that nozzles 109 are placed at intervals along lengthL of the bed.

As illustrated, mixture M flows slowly across the bed to exit on theopposite side and flow into adjoining greenhouse 200 as the ultimate,maximum-concentration, mixture U. Depending on the ratio of L to W, theflow of mixture M could alternatively be in the length direction.Additional nozzles are placed at intervals across the bed to furtherpromote algae growth as its concentration increases. The number of VGAnozzles required is also a function both the bed width and length.Ambient air A is drawn into air spaces 106 and 107 by an externalinduced draft blower, to be solar-heated as it flows across the bed, andis thence delivered into greenhouse 200. Atomizing air flows, C and 5,plus gas mixture G, warmed and humidified in greenhouse 100, flow intogreenhouse 200 to merge with heated ambient air A. The small portion offine droplets in plume P that have not settled back into bed 101 iscarried with it.

Inasmuch as the efficiency of photosynthetic absorption of solar energyis relatively low (generally estimated at 11% maximum), the flow ofambient air A through spaces 106 and 107 serves to absorb excess solarenergy, thereby preventing overheating of greenhouse 100 and bed 101. Ifadditional heat removal is required, algae mixture M can be externallycirculated through a simple pipe-array, external water spray heatexchanger. Maximizing the growth rate and concentration of algaerequires control of the temperature of mixture M in bed 101, preferably(for certain species of micro-algae) to within the range 68° to 72°. Italso requires that the droplet size and solar exposure time of spray Pbe controlled and varied as needed to promote optimum growth while thealgae culture continues to increase in concentration. Since growth ofalgae is a function of the relative periods of light and darkness,successive exposures to sun light, air and CO₂ through repeatedspraying, variation of the quantities sprayed and variation of depth 0of the algae bed are utilized to promote maximum growth rate and algaeconcentration. The effect of the relative humidity of the atmosphere incontact with sprayed algae depends upon the droplet size, dropletexposure time and the algae specie. Since a relative humidity aboveabout 80% is generally preferred, and most desirably above 85%, it isdesirable to limit the influx and exit of air in the greenhouse spaceused for the algae spraying and solar exposure.

Pertinent features of greenhouse 200 include algae bed 201, containingconcentrated algae mixture U, roof covering 202, interior divider 203,atomization space 204, heating and evaporating space 205, particlesettling space 206, bag type solids collector 207 and rear structuralwall 208. The rear wall is preferably finished with a light reflectinginterior surface. Concentrated algae mixture U is delivered by pumps 209to linear VGA nozzles 210, which utilize compressed air C (generallycompressed to the range of 30 to 70 psig.). Nozzles 210 are generallysimilar to nozzles 109 (without the provision for adding air-CO₂mixture), but are designed specifically for fine atomization. Withadjustment features that allow considerable variation in both dropletsize and flow rate, maximum evaporative drying can be produced duringexposure to the available solar energy. Solar-heated ambient air A,flows into atomization space 204 and mixes with air issuing from nozzles109 and 210, plus residual, unabsorbed CO₂, then flows upward throughdrying space 205 carrying the finer droplet size portion of the sprayproduced by nozzles 210, plus any carry-over from nozzles 109. Theupward flow of air and spray droplets causes a fractionation of thegenerally broad distribution of droplet sizes produced by an airatomizer, with the finer fraction being lofted upward. The remainingdroplets (generally larger mass-fraction of the droplets in thedistribution of droplet sizes within a spray) fall back to the bed to bere-atomized. Air stream A, thence flows out of the top of the dryingspace and downward carrying the dry particulate for collection in bagtype filters 207. Air stream A, humidified by evaporation of water fromdroplets during drying, flows from filter 207 out of greenhouse 200 to aheat exchanger consisting of a pipe array cooled by an external spray ofwater delivered from a natural water source. Condensate from the heatexchanger is collectible as desalinated water. Air flow through thegreenhouse enclosures is produced by an induced draft fan following theheat exchanger. Any dissolved salts present in the algae suspension willbe collected together with the dried algae in greenhouse 200. This maybe undesirable, particularly with marine algae where the saltconcentration exceeds that of the algae. In such case, an alternativemethod of operation may be employed. By first delivering theconcentrated algae from greenhouse 100 to an algae separation step suchas centrifuging, the separated solution may then be desalinated ingreenhouse 200 for salt and/or remaining nutrient salts collection. Thesizes of the greenhouses required are estimated from available publisheddata on algae growth, as follows:

Algae Growing Greenhouse

Solar Energy (U. 5.24 hour daily average): 22 W/ft²=1.25 Btu/minute/ft²

Efficiency of Photosynthesis: 7.7%=70% of 11% theoretical max.

Energy Required for Photosynthesis: 114.3 kCal/mol CO₂=3811 kCal/kg=(6mols CO₂=1 mol Algae) 6860 Btu/lb Algae

System Unit Design Basis: 1 gpm of aqueous suspended algae mixtureharvested

System Unit, Harvested Algae Concentration: 2% by wt.=0167 lb/min.=10lb/hr.

System Unit Solar Panel Area for Algae Growth at 10 lbs/hr and at 7.7%Efficiency:

-   -   68600/1.25/60/.077=11900 ft²        Algae Harvesting Greenhouse

To evaporate 1 gpm of water into air heated to 140° F., sat'd., from 70°F., sat'd:

-   -   1247 Btu/lb evaporated or 10400 Btu/gal.

The quantity of air involved: 7.2671 b air/lb water or 800 ft³/gal

Unit Solar Panel Area for Algae Harvest: 10400 Btu/gal/1.25Btu/min/ft²/=8300 ft²

Combined Greenhouse Growing and Harvesting

To completely evaporate finely atomized droplets requires a heated airstream of volume and velocity sufficient to loft them up through thedrying space without their settling by gravity before drying andcollection of the suspended solids. Since this, carrier-air volume issignificantly larger than that required to contain the evaporated water,additional solar panel area must be provided for heating the carrierair. In the present system design, the additional air volume needed toloft the finely atomized droplets is pre-heated by absorbing the 92% ofsolar energy not utilized in algae growth. This is accomplished byproviding the separate air passageway through the double solar panelroof on the algae growing greenhouse. The flow of air in the airpassageway above the culture bed serves the added purpose of preventingoverheating of the bed by absorbing the excess solar heat that is notutilized in growth. For convenience in construction and operation, theadjoining beds are made equal in length. The required bed sizes, basedupon equal solar panel sizes is estimated by the following simplifiedheat balance equation based on 1 gpm algae mixture feed:Q _(S) =Q _(F) +Q _(G) +Q _(A)Q _(S) =Q _(E) +Q _(H)

Q_(S)=Solar energy available=1.25 Btu/ft²/×A_(p), where A_(p)=panelarea, ft²/gpm

Q_(F)=Heat to warm the feed=w_(f)×C_(p)×(70° F.−t_(f)) where t_(f)=feedtemp., w_(f)=8.34 lb/gal feed, C_(l)=specific heat of liquid=1.0Btu/lb/deg. F., and t_(f)=algae feed temp. assumed=60° F.

Q_(G)=Heat absorbed in algae growth=6860 Btu/lb×0.167 lb/min=1146Btu/min

Q_(A)=Heat for added air and CO₂=w₁×C_(a)×(t_(i)−70° F.), where

-   -   C_(a)=specific heat of air=0.25 Btu/lb/deg. F.    -   w₁=w_(a), lbs/min of ambient air+w_(n1), estimated at 3 lbs/min,        air and CO₂ added with nozzles in algae growing greenhouse    -   t₁=the intermediate temperature to which added gases entering        harvest bed are heated

Q_(E)=Heat to evaporate fine droplets=10400 Btu/gal

Q_(H)=Heat added to additional air provided to carrydroplets=w₂×C_(p)×(140° F.−t_(i)),

-   -   where w₂=w₁+wN₂, estimated at 40 lbs/min., nozzle air added for        fine atomization.

With the solar panel areas of the two greenhouses designed to be ofequal length, and set at 12,000 ft² each, and the panel widths assumedto be 40 ft, the bed lengths are 300 ft. Allowing a 6″ channel width ofthe air drying passageway, it is estimated that an air flow rate ofabout 10000 ft³/min will carry droplet of 25-30 microns diameter. Underthese conditions, the air will be preheated to around 140° F. Thecombined footprint area of the two green houses is approximately 83% ofthe solar panel area or 20,000 ft².

In order to accommodate the extended bed length, a multiplicity ofminiaturized, small flow capacity, VGA nozzles are employed. These aremounted in pipe-lance type enclosures suitably spaced at intervals alongthe bed. The lances are fed by pumps that draw the algae suspension fromlocations in the bed selected to maximize circulation of the mixture.The solar energy unused, and thereby wasted, in photosynthesis isutilized for preheating the drying air. This significantly reduces thesolar panel area for harvesting that would otherwise be required forheating the air volume needed to fractionate the droplet sizedistribution and convey the finer droplet sizes. Alternative methods ofevaporating the large amount of water carried with the algae suspensions(typically concentrated to only 2% in current production practice)inherently involve considerable, costly energy.

Desalination

It is noted that essentially the same greenhouse configuration asillustrated in FIG. 1 may also be employed for desalination. In suchcase, the greenhouse identified as 100 is used to preheat the salt waterand air used to loft the fine droplets for evaporation in greenhouse200. It may also be used with brackish and waste water. In alldesalination applications, the feed water is first filtered to removeundesirably large particulate. In the alternative, desalination mode ofoperation, greenhouse 100 is utilized to preheat both air and sea waterprior to evaporation in greenhouse 200. Condensation of the evaporatedwater is accomplished by cooling the moisture laden air by passagethrough an array of pipes externally cooled by spraying with the same,ambient temperature water source as for desalination. It is recognizedthat the efficiency of external spraying depends not only on the watertemperature but also on the ambient air temperature and humidity.However, since the heat transfer is a function of the ambient wet bulbtemperature, it requires less surface pipe surface area than does aconventional shell and tube heat exchanger, which, in fact, isconsidered to be impractical in this application.

Based on a similar heat balance for the same greenhouse design, thedesalination capacity is estimated at 6 gpm per acre.

Carbon Dioxide Extraction

FIG. 2 shows a plan view and elevation view, A-A, of an assembly of amodified flue gas duct and a greenhouse, generally designated by the 300series of numerals, as employed herein for extraction of CO₂ from fluegas. Flue gas 301, after scrubbing to remove 50₂, NO_(x) and mercurymust be cooled, preferably to below about 125° F. This may be done byexternally spray cooling or submerging in a stream or other water sourcea section of duct 302. Pre-cooled flue gas 303 then passes into modifiedflue gas duct 304 fitted with bed 311 containing scrubbing medium 312.Although, as herein suggested, medium 312 would consist of magnesiumhydroxide, Mg(OH)₂, slurry because of its apparent reasonable price andavailability as a waste product, other chemicals could also beconsidered. Medium 312 is repeatedly sprayed into flue-gas-containingduct space 313 with linear, variable gas atomizing nozzles installed innozzle-lances 314. The length of duct 304 provides the time needed forthe CO₂ to diffuse into the extended liquid surface area but a means ofdissipating the heat of reaction evolved between and CO₂ in formingmagnesium carbonates. The liberated heat may be absorbed either byexternally spraying the duct or by submerging in a stream or other watersupply. Cleaned flue gas 305 is released to the atmosphere. Reactedslurry 306 is circulated into greenhouse 307 fitted with bed 315containing circulating slurry 316. Additional nozzles 314 repeatedlyspray slurry 316 into air space 317 where energy received through solarpanel 318 furnishes the heat needed to reverse the reaction and releaseCO₂. Restored Mg(OH)₂ slurry 308 is recirculated back to duct 304 forreuse. Released CO₂ 309, together with the H₂O involved in the reactionis delivered for collection.

The greenhouse size required to extract the CO₂ absorbed by the VGAinduct spray-scrubbing method is estimated as follows:

-   -   Reversible reaction: Mg(OH₂+2 CO₂        Mg(HCO₃)₂    -   Heat of Reaction with CO₂=375 Btu/lb CO₂, exothermic    -   Heat of Reverse Reaction=″ ″ ″, endothermic    -   Carbon Dioxide @14% of Flue Gas=2200 lb/hr/MW    -   Solar Energy Available: 22 W/ft²=75 Btu/hr/ft²    -   US daily average hours of sunlight=4 hrs.    -   Solar Panel Area Required for 100% CO₂ extraction:    -   2200×375/75×24 hrs/day/4 hrs, avg.=66,000 ft²/MW or 1.5 acre per        MW    -   At 16.7% CO₂ removal, or 4 hr/day operation, ¼ acre per MW is        required.    -   The slurry absorption bed required is estimated to be about the        same size.        These and all such other variations which would be obvious to        one skilled in the art are deemed to be within the spirit and        scope of the appended claims where expressly limited otherwise.

In summation of the present disclosure, method and means are describedthat constitute systems for utilizing solar energy to facilitate thefollowing processes: 1. Grow, concentrate, dry and collect micro-algaefrom fresh water, brackish water or sea water as a source of bio-fuel orindustrial products; 2. Desalinate sea, brackish or waste water as asource of non-potable water for industrial use; 3. Extract carbondioxide from flue gas. The method employs two types of modifiedgreenhouses, one type for growing algae and/or preheating air andaqueous liquid mixtures, and the other type for harvesting and dryingalgae or other finely dispersed solids content of slurries by fineatomization and solar evaporation of the water content. The processesare controlled and optimized by employing a variable degree ofatomization via linear type, variable gas atomization nozzles. In onegreenhouse, the nozzles spray the aqueous suspensions into broad andnarrow spray plumes in the form of thin liquid sheets and coarse dropletsizes that rapidly absorb the solar energy. The spray characteristicscan be adjusted by means of the design features of the linear nozzles tomatch the solar radiation. In application to growing algae, the degreeof liquid atomization, together with a temperature controlled and CO₂enriched greenhouse atmosphere, is employed to maximize the growth rateand concentration of the algae. The formation of coarsely atomizeddroplets in a controlled atmosphere, above a bed of nutrient enrichedalgae solution, plus a rapid settling of spray droplets coupled withrepeated spraying, is employed to produce the intermittent andcontrolled period of solar exposure, plus the degree of carbon dioxideintermixing, needed to promote optimum algae growth rate. As the culturesolution reaches its self limiting maximum concentration, it iscontinuously transferred to a second, adjoining greenhouse utilized forharvesting the algae as a dried product. In the second greenhouse,variable, linear nozzles are employed to produce a spray of atomizedalgae solution with a distribution of fine droplet sizes. The finestdroplet size portion of the droplet distribution of the atomized sprayis rapidly heated in a rising air stream, evaporated and dried throughexposure to solar energy. The larger droplets settle back into the algaebed to be continuously re-atomized. The particles of dried algae,expended nutrient and other salts (where sea or brackish water is usedas a source) are collected in a bag house type filter. The heated andhumidified air is cooled to condense its water content by deliverythrough a simple heat exchanger consisting of an array of piping cooledby applying an external water spray. The method and greenhouses are alsoutilized for solar desalination of water and for extraction carbondioxide coupled with its absorption in magnesium hydroxide slurry.

Thus it can be seen that the present invention to provide a novel methodfor growing micro-algae under natural conditions, which method is ofsubstantial benefit from both ecological and also energy-utilizationstandpoints. The invention additionally provides a method having theforegoing features and advantages, which is augmented so as tofacilitate the harvesting of macro-algae product in a highly desirablemanner.

1. In a method for growing micro-algae, the steps comprising: providinga greenhouse that is generally rectangular, viewed in plan, and havingroof structure that is also generally rectangular, that is transparentto solar radiation, and that is of double-pane construction to define atleast one channel through which ambient air can pass to be heated byabsorption of solar energy, the greenhouse containing open-topcontainment means, comprised of at least one receptacle, for thecontainment of a substantially continuous liquid bed and having an inletadjacent one end and an outlet adjacent an opposite end, the containmentmeans being spaced a substantial distance beneath the transparent roofstructure and extending along substantially the full length and widththereof, with the greenhouse defining an enclosed space thereabove;introducing into the containment means a quantity of an aqueous liquidmedium that contains micro-algae organisms and is suitable for growingmicro-algae therein, the quantity of said aqueous liquid mediumintroduced providing a bed that fills the containment means to a depthsufficient to provide a subsurface bed region that is dark, relative tothe surface of said bed, and that extends at least along substantiallythe full length of the containment means; at least periodically addingto the containment means, at the inlet, a fresh supply of said aqueousliquid medium and withdrawing, at the outlet, a volume of said aqueousliquid medium in which the concentration of micro-algae has beenincreased substantially from the concentration of micro-algae in saidfresh supply of said aqueous liquid medium; repeatedly or continuouslydrawing quantities of said aqueous liquid medium from said subsurfaceregion of said bed and spraying said quantities of aqueous liquid mediuminto the enclosed space, at numerous locations spaced longitudinallyfrom one another, using a multiplicity of nozzles that are constructedto enable characteristics of the spray discharge to be varied by meanscontrolled remotely from the nozzles, the nozzles effecting discharge ofsaid aqueous liquid medium from positions above said bed surface and inthe form of thin, substantially flat sheets that are orientedsubstantially horizontally or at a small angle of inclination relativeto horizontal; causing ambient air to flow through the at least onechannel of the roof structure so as to permit said air to absorb asubstantial portion of the solar radiation impinging on the roofstructure and thereby to produce a supply of heated air exitingtherefrom; and controlling said spray discharge characteristics, thequantity of said aqueous liquid medium sprayed, and the duration andfrequency of said continuous or repeated spraying, for optimization ofthe exposure of micro-algae in, and drawn from, said bed to solar energypassing through the roof structure and to induce mixing of said aqueousliquid medium in said bed, thereby promoting micro-algae growth and, inturn, increasing the concentration of micro-algae in said bed, controlof said spray discharge characteristics being such as to cause at leastabout 90 weight percent of said aqueous liquid medium issuing from thenozzles to return to said bed.
 2. The method of claim 1 wherein saidangle of inclination at which said substantially flat sheets of spraydischarge from the nozzles is about zero to 20 degrees, and wherein saidsheets are about 0.01 to 0.1 inch thick.
 3. The method of claim 1wherein said bed of aqueous liquid medium is about one to four feetdeep.
 4. The method of claim 1 wherein said subsurface bed region liesat least about 0.25 inch below said bed surface.
 5. The method of claim1 wherein the concentration of micro-algae organisms in said quantity ofsaid aqueous liquid medium introduced into the containment means, and insaid fresh supply of said aqueous liquid medium added thereto, is about0.01 weight percent.
 6. The method of claim 1 wherein said concentrationof micro-algae in said volume of said aqueous liquid medium withdrawnfrom said containment means is about two percent by weight.
 7. Themethod of claim 1 wherein said droplets returning to said bed consistessentially of droplets having diameters of at least about 20 microns.8. The method of claim 1 wherein said flow of ambient air through the atleast one channel of the roof structure is controlled so as to controlthe amount of solar energy that pass through the roof structure foroptimization of micro-algae growth in the greenhouse.
 9. The method ofclaim 1 wherein air and carbon dioxide are introduced into the enclosedspace above the containment means, directly and/or through themultiplicity of nozzles contained in the greenhouse.
 10. The method ofclaim 1 wherein at least one nutrient, for promotion of micro-algaegrowth, is introduced into said bed of aqueous liquid medium.
 11. Themethod of claim 1 wherein the temperature of said aqueous liquid mediumin said bed, and of the environment within said enclosed space, iscontrolled to about 60° to 120° Fahrenheit.
 12. The method of claim 1wherein the relative humidity in the environment within said enclosedspace is maintained at a value of at least about 80 percent.
 13. Themethod of claim 1 wherein said method enables harvesting of amicro-algae product and includes a further step of evaporating freewater from micro-algae contained in said volume of said aqueous liquidmedium withdrawn from the containment means, said supply of heated airexiting from the at least one channel of the roof structure of thegreenhouse being employed in effecting said evaporation step.
 14. Themethod of claim 13 wherein a second greenhouse is provided in which saidevaporating step is effected, the at least one channel of the roofstructure of the first-mentioned greenhouse being operatively connectedto the second greenhouse for the delivery of said supply of heated airthereto.
 15. The method of claim 14 wherein the second greenhousecontains a second multiplicity of spray nozzles supplied by said volumeof aqueous liquid medium withdrawn from the outlet of the containmentmeans, and wherein said supply of heated air is employed for loftingparticles discharged from the second multiplicity of spray nozzles, nomore that about 50 weight percent of such discharged droplets havingdiameters larger than about 40 microns.